The Cd4 protein serves as the major protein receptor for the human immunodeficiency virus (HIV). Expression constructs of the first 104 amino acids of the protein (the V1 domain) can bind with high affinity to the gp120 chain variable domain and has been used to classify the CD4 protein as a member of the Ig-superfamily of proteins. Mutational data suggests that a small region within the V1 domain of the CD4 protein (residues 38- 55) is responsible for the observed binding of this domain to the gp120 protein. The region 38-55 covers a subdomain of the CD4 protein that corresponds to one of the three antigen-antibody contact structures within the light chain variable domain (the three antibody contact domains are referred to as CDR1, CDR2 and CDR3). Our preliminary results provide the first direct evidence that the CDR2-like domain of the CD4 protein (residues 45-54)are directly involved in contacting the gp120 protein. Furthermore, we have evidence that the CDR3 domain (CD4 residues 84-101) is also involved in this contact as well as suggestive evidence of the involvement of the CDR1-like domain. Using a known light chain variable domain structure as a folding template, we have modeled the V1 domain of the CD4 protein. We have used our CD4 model as a structural template for the design of conformationally-restricted synthetic peptides. Our preliminary work with both the CDR2-like domain of the CDR2- and CDR3-like domains of the CD4 protein were performed using these restricted analogs. We have shown that peptides derived from both of these restricted analogs. We have shown that peptides derived from both of the CDR-like domains are capable of potently inhibiting virus binding to CD4 positive cells. Here we propose to expand our use of the conformationally-restricted analogs to identify CD4/gp120 contact regions as well as refine such structures to yield a panel of high affinity reagents capable of preventing the HIV-CD4 interaction. Based on our preliminary results and the known structure/function relationships within the Ig-superfamily of proteins, we propose that the gp120 contact surface on the CD4 protein is directly analogous to the antigen contact surface on an antibody. Finally, in order to prove (or disprove) and extend this hypothesis, we propose to perform protein footprinting experiments on the CD4/gp120 complex. This will identify protein regions sequestered when the protein-protein complex is formed.